For about a half a century it has been known that some aromatic disulfides are effective in the polymerization of ethylenically unsaturated organic compounds, but to this date, not much is known about how, why, and under what conditions they are effective, if at all they are.
This invention is particularly related to novel disulfides distinguished by their intra-ring linkage, and their use in photosensitive materials (commercially available under the Cycolor* trademark) of the general type comprising a laming support having a layer of microcapsules on one surface thereof. Encapsulated is a photosensitive free radical, addition-polymerizable composition containing an autooxidizer which functions as a co-initiator to enhance the speed of a polymerization reaction which is triggered by absorption of actinic radiation. The term "actinic radiation" as used herein includes, but is not limited to, visible light, ultraviolet radiation, and infrared radiation. Enhanced speed of polymerization results in improved film speed. An autooxidizer consumes oxygen which inhibits the speedy propagation of chains in a free radical addition polymerization process.
More specifically, the film speed at which there is detectable change in the hardness of the composition (that is, the shoulder speed in a positive working composition) is a product of the quantum efficiency whit which the photoinitiator system generates free radicals and the quantum efficiency with which the incipient radicals react with oxygen. To maximize film speed, both efficiencies must be maximized.
It is well known that molecular oxygen strongly inhibits the radial initiated polymerization of acrylate monomers which typically display a `lag` (induction time) prior to the onset of polymerization. Kinetic measurements have shown that during the induction time, the concentration of oxygen in acrylate monomer declines due to oxidative reactions, and no detectable polymerization begins until the concentration of oxygen is reduced to a few tenths of a percent of the equilibrium value. Thus, the photographic sensitivity of acrylate photopolymerization systems is governed in large part by this photooxidation process.
We known of nothing in the prior art that implies that there may be some rational, logical basis for choosing an autooxidizer which will provide any desired film speed. It is known however, that dyestuff/redox photohardenable compositions are peculiarly sensitive to the choice of autooxidizer, and that with an ionic dye/reactive counter ion complex, and specifically a cationic dye/borate anion complex, N, N-dialkylanilines function as effective autooxidizers. This has been disclosed in U.S. Pat. Nos. 4,772,530; 4,842,980 (class 430/subclass 138) and 4,772,541 (class 430/subclass 339) to Gottschalk et al. The photographic process based on the photopolymerization of acrylate monomers is described in U.S. Pat. Nos. 4,399,209 and 4,842,981 (class 430/subclass 138) to Sanders et al; and, in U.S. Pat. No. 4,840,866 (class 430/subclass 138) to Kovacs et al. The disclosures of each of the foregoing patents, all of which are commonly assigned to The Mead Corporation, are incorporated by reference thereto as if fully set forth herein.
In the photographic process described in the '209 and '836 patents, the photosensitive monomer composition is encapsulated with a photoinitiator and an image-forming agent. Exposure of the imaging sheet hardens the internal phase of the microcapsules and renders them resistant to physical rupture. Thus, a latent image is formed as an image-wise pattern of hard (exposed) and soft (not exposed) microcapsules. The visible image is developed physically by bringing the exposed capsule sheet into contact with a receiver sheet under pressure. Only the soft capsules rupture and deliver the image-forming agent, dissolved in the monomer, to the receiver sheet. The result is a direct positive of the original image. It has been shown that the exposure required to produce the first detectable loss in image density in the microencapsulated acrylate system is a function of the efficiency of photo-oxidation.
Several prior art references disclose that disulfides are effective autooxidizers, but they disclose particular disulfides which presumably are effective enough in their systems, but have proven to be far from adequately effective in an ionic dye/reactive counter ion system. We attribute the effectiveness of our disulfides to a unique "fingerprint" linkage which has the structure. ##STR2## wherein X is S or O excerpt for a specific case as discussed below. The fingerprint linkage is part of an aromatic ring with the sulfide S atom being exocyclic. This "fingerprint" linkage must be present in at least one of the two sulfide radicals which are directly single-bonded in the disulfides found effective in our photosensitive composition. In a specific case, as mentioned above, where one or both sulfide radicals are derived from 2-mercapto-tertrazole, the X in the fingerprint linkage is N. Typically, the disulfide linkage in our novel compounds directly connects aromatic groups in first and second sulfide radicals, and the aromatic groups in the sulfide radicals are generally different, but may be the same. Though certain of the novel disulfides in which the electron-withdrawing effect of the --S--S-- connected aromatic rings is attenuated by a methylene group in each sulfide radical, are photoinitiators in unencapsulated photosensitive compositions, they are not effective in our photoinitiator system.
Implicit acknowledgement that the linkage to the exocyclic S atoms is critical, is found in U.S. Pat. No. 4,304,841 to Horn et al (Class 430/subclass 286) which disclosures disulfides having a --CH.sub.2 --S--S--CH.sub.2 -- linkage which is calculated to attenuate the electron-withdrawing effect even of aromatic rings. Such disulfides are used in a photopolymerizable mixture which contains a polymeric binder, a compound which is polymerizable by free radical mechanism, and a photoinitiator. The disulfides are represented by the formula EQU R.sub.1 --CH.sub.2 --S--S--CH.sub.2 --R.sub.2
wherein R.sub.1 and R.sub.2 are identical or different and are alkyl, cycloalkyl, aryl, aralkyl or N-substituted carbamoyl or carbamoylalkyl groups. The initiators used are acridine, phenazine, quinoxaline, quinazolline, benzalquinaldine and thiopyrylium.
The general notion that an aromatic disulfide, that is, one in which the disulfide linkage connects aromatic radicals, was a useful co-initiator in a free radical polymerization was disclosed by Richards in U.S. Pat. No. 2,460,105 (Class 204/subclass 162) more than four decades ago. His primary source of free radical initiation was ultraviolet light generated by a 100 watt mercury lamp over more than 24 hr at a temperature above room temperature. He also disclosed that alkyl disulfides, cycloalkyl disulfides, acyl disulfides and thiuram disulfides displayed the activity he sought. This activity was characterized by the ability to polymerize styrene irradiated with the mercury lamp.
Two decades later, Rudolph et al, in U.S. Pat. No. 3,450,612 (Class 204-subclass 159.15), disclosed the use of aromatic disulfides in combination with peroxides as coinitiators for a free radical polymerization. A decade after Rudolph et al, Donald et al in U.S. Pat. No. 4,168,981 (Class 96/sublcass 115 P) disclosed that bis(substituted amino)polysulfides were useful coinitiators.
None of the prior art disulfides disclosed are directly single-bonded sulfide radicals each of which has the unique fingerprint linkage of our invention which must be present as part of an aromatic ring having 5 or 6 ring members. The ring may be fused to another, with the sulfide S atom being exocyclic, provided that the aromatic ring in each radical is not tetrazole. By "aromatic" we refer to a pi-bonded ring system containing 4n+2 pi electrons where n is 1 or 2.
Despite the commercial success of the ionic dye/reactive counter ion initiator in a photosensitive composition, and particularly one used as an internal phase of microcapsules, using an autooxidizer which attacks the acidic cationic dye/borate anion initiator decreases the shelf life of photosensitive material in which the system is used.
In particular, Japanese provision publication Tokkaisho 64-13144 to Yamaguchi et al (laid open Jan. 18, '89) discloses the use of thiols some of which have the fingerprint ring structure and bis-compounds of the thiols. In the disclosed bis-thiols, the connecting bridge is alkylene, and such bis-thiols are not disulfides. The aromatic thiols with the fingerprint structure have been found to be highly effective to improve speed with a cation dye/borate anion complex, but Yamaguchi et al fail to indicate that the complex is sensitive to, and deleteriously affected by the acidic portion in the thiols. The portion attacks the carbon-boron bond and the complex dissociates. The destruction of the borate ion causes a corresponding reduction in the amount of photoinitiator present. Thus the "shelf-life" of the photosensitive composition is reduced.
Shelf-life, typically, is not a consideration in photosensitive compositions for planography plates, resin relief printing plates and resists or photomasks for making printed circuit boards, but shelf-life is an overriding consideration for a donor sheet or cartridge stock in a color photocopier, or in other comparable applications where preservation of the complex is essential. Nor does the reference intimate that their thiols and bis-thiols would exhibit much greater sensitivity after they are encapsulated than before, as do our disulfides. Further, the Yamaguchi et al compounds relate both to monoaza and diaza thiols in which the substitution of a C atom or a N atom respectively, to replace S or O in the aromatic ring of the thiol, have a known different electron withdrawing effect on the --SH group. When they couple thiols, they require a spacer between the coupled S atoms.
Accordingly, there exists a need in the art for an autooxidizer which (i) does not decrease the shelf life of film in which the autooxidizer is used, and (ii) may be used in a photosensitive composition which includes an ionic dye/reactive counter ion photoinitiator system, and more particularly to a microencapsulated photosensitive composition which is particularly sensitive to light in the visible wavelength range.